1. Field of the Invention
The present invention relates to an apparatus for testing the tensile strength of sheet material and, more particularly, to an improved clamping jaw assembly which is adapted to transfer compression forces directly to the sheet material to be tested.
2. Description of the Prior Art
U.S. Pat. Nos. 3,707,119, issued Dec. 26, 1972, and 4,059,992, issued on Nov. 29, 1977, to Wavell F. Cowan, disclose testing devices for measuring the tensile strength of sheet material, such as paper. Basically, these testing devices comprise pairs of opposite clamping jaws which are respectively adapted to clamp a portion of the sheet material adjacent to the portion clamped by the opposed pair of clamping jaws. One of the opposed pairs of clamping jaws is adapted to move away from the other between a first position whereby the two pairs of opposite clamping jaws are juxtaposed in contact with each other and a position spaced apart from each other with the clamping jaw still gripping portions of the sheet. Means are provided for pivotally causing the movement of one pair of clamping jaws relative to the other.
Each pair of clamping jaws of the testing device described in U.S. Pat. 4,059,992 is provided with a fixed jaw member and a moveable jaw member. Low pressure, large diameter, pneumatic cylinders are provided for moving the moveable jaw members towards the fixed jaw members against a self-aligning cylindrical spring of each pair of clamping jaws.
Although the testing devices described in the above-mentioned patents are very effective, it has been found that some deflection still affects the uniformity of the clamping surfaces of the fixed and the moveable jaw members. Indeed, according to these constructions, some bending forces are induced in the moveable jaw members. In some applications, eliminating even very small deflection at the clamping surfaces is hugely beneficial to the accuracy of the tensile test.